> No Lambda costs as it would be covered by the monthly free tier of 1M requests.

That's far from the full picture. AWS Lambdas are charged by units of computational resources that are expressed as multiples of 64MB of RAM used per 100ms, each rounded up to the next value and with a minimum charge of 128MB of RAM used. So you are only charged a fixed fee per request if all your requests are short-lived and barely use any computational resources. Long-lived processing tasks that require a bit of RAM are charged multiple times the value of a single request.